Asymmetrical Diruthenium Complex Bridged by a Redox-Active Ligand

The asymmetrical dinuclear complex [(acac)₂Ru1­(μ-abpy)­Ru2­(Cym)­Cl]­PF₆ ([<b>2</b>]­PF₆), with acac^– = acetylacetonato = 2,4-pentanedionato, abpy = 2,2′-azobis­(pyridine), and Cym = <i>p</i>-cymene = 1-isopropyl-4-methylbenzene, has been obtained from the mononuclear precursors [Ru­(acac)₂(abpy)] and [Ru­(Cym)­Cl₂]₂. X-ray crystal structure analysis suggests the oxidation state formulation [(acac)₂Ru1^III(μ-abpy^•–)­Ru2^II(Cym)­Cl]⁺ for <b>2</b>^<b>+</b>, with antiferromagnetic coupling between one Ru^III center and the radical-anion bridging ligand (abpy^•–), based on the N–N distance of 1.352(3) Å. As appropriate references, the newly synthesized mononuclear [(abpy)­Ru^II(Cym)­Cl]­PF₆ ([<b>1</b>]­PF₆) with an unreduced NN double bond at <i>d</i>(NN) = 1.269(4) Å and the symmetrical dinuclear [(acac)₂Ru^2.5(μ-abpy^•–)­Ru^2.5(acac)₂] with <i>d</i>(NN) = 1.372(4) Å (rac isomer) support the above assignment for <b>2</b>^<b>+</b> as an asymmetrical mixed-valent configuration bridged by a radical ligand. Reversible one-electron oxidation leads to a dication, <b>2²⁺</b>, with largely metal-centered spin (EPR: <i>g</i>₁ = 2.207, <i>g</i>₂ = 2.155, and <i>g</i>₃ = 1.929), and a weak intervalence charge-transfer absorption at 1700 nm, as observed by spectroelectrochemistry. These results support a description of <b>2²⁺</b> as [(acac)₂Ru1^III(μ-abpy⁰)­Ru2^II(Cym)­Cl]²⁺. Density functional theory (DFT) calculations suggest that the first reduction of [<b>2</b>]­PF₆ also involves the bridging ligand, leading to [(acac)₂Ru1^III(μ-abpy^2–)­Ru2^II(Cym)­Cl] (<b>2</b>). Experimentally, the first reduction of <b>2</b>^<b>+</b> is not fully reversible, with evidence for the loss of chloride to form [(acac)₂Ru1­(μ-abpy)­Ru2­(Cym)]⁺ (<b>2a⁺</b>; <i>g</i>₁ = 2.454, <i>g</i>₂ = 2.032, and <i>g</i>₃ = 1.947). Further reduction produces [(acac)₂Ru1^II(μ-abpy^2–)­Ru2^II(Cym)] (<b>2a</b>), which forms [(acac)₂Ru1^II(μ-abpy^2–)­Ru2^I(Cym)]⁻/[(acac)₂Ru^II(μ-abpy^•–)­Ru⁰(Cym)]⁻ (<b>2a^–</b>) in yet another one-electron step (<i>g</i>₁ = 2.052, <i>g</i>₂ = 2.008, and <i>g</i>₃ = 1.936). The major electronic transitions for each redox state have been assigned by time-dependent DFT calculations.